Amplifier system



Dec. 18, 1934. H, DRESSEL 1,984,967

AMPLIFIER SYSTEM Filed March 20, 1933 25 i 38 wi INVENTOR ATTORNEY Patented Dec. 18, 1934 AMPLIFIER .sYsfrEnr Henry Dressel, Chicago, 111., .asslgn'or Grigsby-Grunow Company, Chicago, Ill., a

corporation of receiver Illinois; ,M. McKey,

Application March 20, 1933, Serial o. 661,763

9' Claims. 01. 179-171) This invention relates to amplifier systems, and particularly to'so-called Class B amplifiers. As is well known, a Class B amplifier comprises a pair of vacuum tubes connected in opposed relationship and operating with little or'no bias on their grids. Hence, signal impulses tend to swing each grid alternately positive, resulting in heavy surges of plate current. While the average load on such an amplifier may not be any greater than in the ordinary push-pull amplifier with negatively biased grids, nevertheless the instantaneous loads may reach values high in comparison with a conventionl push-pull amplifier. With the customary power supply, comprising a transformer feeding into a rectifier and a suitable filter system,

it is clear that the substantial variation in load' drawn by the Class B amplifier will have adeleterious effect on the output potential of the power supply. Such power supply systems invariably have poor regulation. The net result of this will be that at maximum volume, when the amplifier tends to draw most current, the voltagewill drop,-

tending to cut down the reproducible volumein the speaker. Hence, volume distortion will be the natural result.

An object of this invention isto devise a system of this character wherein such distortion. is either eliminated entirely or gible value. p

The single figure :isa diagrammatic represen tation of a system embodying this invention.

A source of audio-frequency currents, such as a radio receiver, is connected to a transformer 1, having a center tapped secondary 2. The center 3 of secondary 2 is connected to a grounded cathode 4 of an amplifier 10. This amplifier may have a heater 11 energizing cathode 4 and a pair of grids 12 and anodes 13 cooperating with said cathode 4. Grids 12 are connected by suitable conductors to the opposite ends of transformer secondary 2. Anodes 13 are connected by conductors 14 to the ends of a primary output transformer 15. A by-pass condenser 16 is connected across conductors 14.

Output transformer 15 has its secondary 17 connected to the voice coil 18 of a dynamic speaker having a cone 29. The dynamic speaker has its field (here diagrammatically shown as a core 21) energized by a coil 22. Coil 22 functions as the choke in the filter system of a power supply.

This power supply comprises a transformer 25 having a primary 26, fed by customary -cycle alternating current. A secondary 2'7 is adapted to energize the cathode 30 of a full-wave rectifier 31. Anodes 32 of the rectifier are connected reduced to a'ne'glito the opposite ends of transformer secondary 35. The mid point of this winding is grounded and is connected through a filter condenser 36 to one end of speaker coil choke 22 by means of a. conductor 37. A conductor 38 also con-' nects a conductor 37 with the cathode 30 of the rectifier.v The other end of coil 22 is connected to the centerof primary of transformer 15 and through a. condenser 23 is also connected to. the grounded center tap of transformer winding 35.

In the design of the system, the dimensions of speaker core 21, together with the magnetizing force of the coil 22 are sorelated that under normal conditions the magnetic fiux density has a value lying on the straight part of the wellknown B- -H curve;

The operation of the system is as follows: Assuming that signal impulses are being fed into transformer 1, said impulses having an amplitude corresponding to a high volume of speaker reproduction. Under these conditions, grids 12. ofsamplifier 10 will'become alternately positive and will greatly increase the conductance of the tube from cathode 4 to one of the corresponding anodes 13. As the plate current rises, the voltage falls ofi,'thus resulting in. a disproportionately decreased. amplitude'in the secondary of zoutput transformer l'land thence inthe voicecoil 18. However, as the current drawn byamp fier 10' increases'in value, itis evident that theimagnetic speakertends to more nearly followa group of -variations of the amplifier input. Condenser 23 functions to supply additional currents for instantaneous increases in plate load, the previously described operation occurring when the increased loadexists for a period longer than condenser 23 can accommodate.

It has been found that with customary transformer designs, if the speaker coil 22 is operated on the straight portion of the BH curve, the compensation thus afforded is substantially perfect, so that speaker volume rises and falls in a measure corresponding to the input of the amplifier. It is obvious, however, that in certain cases, it may be desirable to operate core 21 of the speaker on a different portion of the magnetization curve.

I claim:

1. In an audio-frequency amplifier, the combitube results in a substantial decrease in the available potential, a speaker of the electro-dynamic type having a magnetizing coil, means for feeding the output of said amplifier to said speaker, and means for feeding said power supply output through said magnetizing coil whereby said coil functions as a filtering inductance.

2. In an amplifying system, the combination of at least one vacuum tube, said-tube having such characteristic and being operated in such a manner that the plate current goes through a change in value under normal operating conditions great in comparison to a tube operated with normal negatively biased grid, a power supply therefor including a source of fluctuating direct current and a filter system, an electro-dynamic speaker, means for energizing the voice coil of said speaker from said amplifier output, said speaker field functioning as the choke in said filter system and being so dimensioned that the flux density therein during normal operation corresponds to the straight portion of the magnetization curve.

3. The system of claim 2 wherein a condenser is connected across the coil and power supply, whereby said condenser supplies additional current for instantaneous increases in load.

4. An amplifying system comprising at least one vacuum tube, said tube having such characteristics that during normal operation thereof the plate current therein goes through a change of values great in comparison to a tube operated with normal negatively biased grid, an input circuit for said tube, an output circuit for said tube, said output circuit including a circuit coupling member, a dynamic speaker, means for conducting the impulses in said coupling member to said speaker voice coil for reproduction, a power supply including a source of fluctuating direct currents, meansfor conducting said fluctuating currents through the magnetizing coil '1 of said speaker for filtration purposes, and means for impressing the filtered current on the'output circuit of said tube whereby as the tube draws more current and the available potential on the output circuit drops the increased current through the speaker field compensates for the decreased amplitude in the voice coil circuit.

5. The system of claim 4, wherein the speaker field is of such dimensions that the magnetic intensity during normal operation lies on the straight portion of the magnetization curve.

6. An amplifying system comprising a pair of vacuum tubes with their input and output circuits in opposed relationship, said tubes being designed to operate with little or no negative bias on their input circuits, thus resulting in great variation of output plate current, a dynamic speaker having a voice and magnetizing coil, means for impressing impulses in the tube output circuits'on the speaker voice coil for reproduction purposes, a power supply including a source of fluctuating direct current, means for connecting said speaker field in the tube output circuits and power supply circuit so that said fluctuating current is filtered and applied to the output electrodes of said tubes whereby the variable field strength in the speaker tends to compensate for any amplitude distortion in the voice coil due to poor regulation of the power supply system. a

7. The system of claim 6 wherein the field intensity of the speaker during normal operation lies on the straight portion of the magnetization curve.

8. The system of claim 6 wherein a condenser is connected from that end of the speaker field having the closest potential proximity to the potential of the tube output electrodes across said field and power supply in series whereby for instantaneous increases in plate currents said condenser will tend to discharge.

9. The system of claim 6 wherein a condenser is connected from that end of the speaker field having the closest potential proximity to the potential of the tube output electrodes across said field and power supply in series whereby for instantaneous increases in plate currents said condenser will tend to discharge, and wherein the field intensity of the speaker during normal operation lies on the straight portion of the magnetization curve.

HENRY M. DRESSEL. 

